Your question is confusing because you seem to be using some terminology incorrectly.
You said "I received an H1B visa." But it's not possible to receive a US visa inside the US (rare exceptions for diplomatic visas which are not relevant here), because you can only get US visas at US consulates abroad, as US visas are solely for entering the US, and you don't need to "enter" if you are already in the US. The "visa" is the physical sticker that is affixed to a page on your passport. When you said "H1B visa has not been stamped", what you actually mean is that you do not have an H1b visa.
What you probably meant was that you got H1b status while in the US, by doing a Change of Status inside the US. Or you meant that your employer applied for an H1b petition for you and it was approved. Because you were in the US when your employer applied for an H1b petition, it almost certainly also requested a Change of Status, so with your H1b petition approval you should have received a new paper I-94 attached with it, indicating H1b status and the period of admission. That paper I-94 is the I-94 you should use from now on, and all previous (electronic or paper) I-94s you may have received are now invalid. By getting the I-94, that means that you are now in H1b status (unless the I-94 indicates a start date in the future), and there is no "activating" or anything else. (You may be confused because you are looking at CBP's I-94 website, which only has electronic I-94s from entries to the US. But I-94s from Change of Status are not going to show up on there as Change of Status is handled by USCIS which is a different agency from CBP and which issues paper I-94s.)